Stand-alone intercom systems are known in the prior art and generally operate independent of the telephone system, each unit of which is independently connected through its own wiring system.
Telephone intercom systems using a portion of an existing telephone network are also known in the prior art as an integral feature in business telephone systems such as PBX and key systems. This feature is integral to the business telephone systems, taking advantage of hardware which is not available to non-business telephone systems, such as in a residential environment.
Intercom systems which are usable in residential environments and which do make use of at least some of the existing telephone networks are also known in the prior art. For example, U.S. Pat. No. 4,588,861 to Teich and U.S. Pat. No. 4,196,317 to Bartelink both show intercom systems which work in conjunction with existing telephone networks in a business or residential environment. These prior art intercom systems, however, take advantage of the two extra conductors of a four-conductor wire typically provided on the premises of the telephone subscriber. These extra telephone wires are not always available in all environments, and thus make the prior art intercom systems unusable in some environments.
In applications where simple announcement of incoming telephone calls is required where multiple telephone sets are connected to a single subscriber's line, an intercom system is useful but two-way voice communication is unnecessary. When one of a plurality of telephones on a subscriber's line is answered, a simple announcement is all that is required to call the intended party to pick up one of the other telephones on a subscriber's line.